1. Field of the Invention
This invention relates to fault tolerant storage systems and storage devices that store data in stacks within an integral storage medium.
2. Description of the Related Art
One goal of storage system developers is to increase the density of storage devices. Currently, many commercially available devices are limited in that they only use the surface of the storage medium for data storage. In order to overcome this limitation, commercial versions of new storage technologies are being developed that use the physical volume of the storage medium for data storage. These storage devices may provide significantly improved storage density over surface-only storage devices.
One volume-storage technique involves storing holograms of pages of data as diffraction gratings. The diffraction gratings are created by shining a data beam that has been modulated by the page of data and a reference beam onto a selected area of the storage medium. The reference and data beams create an interference pattern that is stored on the selected area of the storage medium as a diffraction grating. The data is retrieved by shining the reference beam onto the selected area, which regenerates the data beam. The resultant data beam is projected onto a CCD (Charge Coupled Display) reader or other photosensitive detector that converts the image into the page of digital data. This storage technique, referred to as holographic storage, offers both improved storage density and improved access speeds. Improved access speeds are possible since data may be accessed in pages instead of in bits. Multiple pages may be stored in a “stack” within the same physical volume of the storage medium by using a different angle and/or phase of the reference beam to access each different storage location within a particular stack.
Another volume storage technique involves integrating multiple surface-only storage media in such a way that a multi-layer storage medium is created. Typically, the resulting storage medium is accessible by a single read and/or write mechanism. For example, in U.S. Pat. No. 5,202,875, multiple light transmissive CDs (Compact Discs) are stacked together to form an integrated, multi-layer disk storage medium. Each layer transmits light so that a laser used to read data can access all of the layers. Each layer may be accessed by focusing the laser on that layer. Data can be stored in “stacks” by storing different units of data to the same position on different layers. The multi-layer volume storage medium provides improved storage density over single layer storage media. Such a multi-layer volume storage medium differs from a conventional set of hard disk platters in that the multiple layers are united into a single storage medium that may be accessed by a single access mechanism. In contrast, conventional hard disk platters are spatially separated and cannot be accessed by the same access mechanism.
Many conventional storage arrays that include multiple surface-only storage devices provide protection against data loss by storing redundant data. Such redundant data may include parity information (e.g., in systems using striping) or additional copies of data (e.g., in systems providing mirroring). The redundant data is stored on a different device than the device on which the data it protects is stored. For example, in a conventional RAID (Redundant Array of Independent Disks) system, an array of disk drives is controlled by a RAID controller (which may be implemented in hardware and/or software). The RAID controller causes redundant data (e.g., a parity block for a stripe or a mirrored copy) to be stored on a different disk than the primary data. In other systems, redundant data may be stored during a back-up operation. For example, a system operator may copy primary data from one or more hard disks to a tape backup system. It is desirable to be able to protect data stored in a stack within a storage medium against data loss.